The image sensor is a device that converts optical signals to digital signals. A general image sensor includes a plurality of pixels and a signal processing circuit. Each of the plurality of pixels includes a photoelectric conversion element and a plurality of transistors. The plurality of transistors are used to transmit a pixel signal generated by the photoelectric conversion element to the signal processing circuit.
The signal processing circuit generates digital signals corresponding to optical signals by processing a pixel signal output from each of the plurality of pixels. A display device displays a digital image corresponding to an optical image in response to digital signals output from the signal processing circuit. An image quality of a digital image is affected not only by a signal processing function of a signal processing circuit, but also by the pixel structure that converts an optical signal to an electrical signal.